Organic high-molecular polymers such as FEP (Copolymer of Tetrafluoroethylene (TFE) and Hexafluoropropylene (HFP)) materials have been used conventionally for electret elements which are dielectric materials each having a permanent electric polarization and applied to devices such as a condenser microphone. However, since these materials are inferior in thermal resistance, the problem has been encountered that they are difficult to use as elements for reflow when mounted on substrates.
As a solution to the problem, an electret using a silicon oxide film as shown in Patent Document 1, instead of an organic high-molecular polymer, has been proposed in recent years to provide a thinner-film and smaller-size electret by using a microfabrication technology.
Specifically, the technology shown in Patent Document 1 deposits a silicon oxide film on a surface of a base, sets a gas atmosphere containing oxygen and containing no moisture in a deposition chamber without releasing the chamber to an ambient atmosphere, performs a thermal process at 200° C. to 400° C. with respect to the silicon oxide film in the atmosphere, and then performs a charging process with respect to the silicon oxide film.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-33241